simple code but i get error C3861: 'ldexpf': identifier not found

[Tolt]

i cant really say that im a total beginner to c++, read 3 ourses in school and alot on my spare time but now when i added FLTK to my visual studio 2010, i get error C3861: 'ldexpf': identifier not found
even with beginner stuff like hello world programs like this one.

#include <iostream>
using namespace std;
int main( )
{
           cout << "Hello World!";
           return 0;
}

and after i added FLTK it worked then all of a sudden i get that error on everything i do >.>